Vertical turbomolecular pumps

ABSTRACT

The vertical rotating assembly of the turbomolecular pump is centered by two magnetic bearings located each on one side of the pump rotor and supported by ball bearings located under the rotor, one on each side of the driving motor. The length of transmission shaft portion between the rotor and the rotating part of the motor is sufficient to compensate for radial deflection of the rotor.

United States Patent [191 Rousseau et al,

[ Nov. 13, 1973 VERTICAL TURBOMOLECULAR PUMPS Inventors: Marcel-Gustave Rousseau, Saint Denis; Robert Berthoumieux, Paris, both of France Assignee: Societe Nationale DEtude Et De Construction De Moteurs DAviation and Commissariat A LEnergie,

Atomique, Paris, l rance Filed: Mar. 25, 1971 Appl. No.: 128,073

Foreign Application Priority Data Apr. 1, 1970 France 7011572 US. Cl 417/424, 415/90, 308/10 Int. Cl. F04b 17/00, F04b 35/04, FOld l/36, F160 35/00, F16c 39/00 Field of Search 417/423, 424; 415/90; 308/10, 194

[56] References Cited UNITED STATES PATENTS 1,402,053 1/1922 Dake 415/90 3,107,310 10/1963 Carriere et a1 308/10 X Primary Examiner-Robert M. Walker Attorney-Lane, Aitken, Dunner & Ziems [57] ABSTRACT The vertical rotating assembly of the turbomolecular pump is centered by two magnetic bearings located each on one side of the pump rotor and supported by ball bearings located under the rotor, one on each side of the driving motor. The length of transmission shaft portion between the rotor and the rotating part of the motor is sufficient to compensate for radial deflection of the rotor.

8 Claims, 3 Drawing Figures PAIENIED unv 13 ms 3; 771. 909 SHEET 3 BF 3 FIGS INVENTORS Passer awzwad/a/e'dx VERTICAL TURBOMOLECULAR PUMPS BACKGROUND OF THE INVENTION The invention relates to turbomolecular pumps of the type having a vertical shaft carrying a rotor rotated at high speed within a housing and formed with disks which draw residual gas from an end of the rotor and move it toward the other end of the rotor connected to a primary vacuum source.

It is one of the main difficulties in the construction of turbomolecular pumps that the rotor (rotating at speeds in the 10,000 rpm range) should be supported andaccurately centered. The problem is still rendered more difficult by, the requirement that contamination of the vacuum by lubricant vapors should be avoided.

In the past, several attempts were made to. solve the problem one of them involves the use of gas bearings but the rate of flow across gas bearings adapted to operate correctly is so high that it has an adverse effect on the performances of the pump. Another solution described in French Pat. No. l 475 765 filed Jan. 31, 1966 makes use of electromagnetic bearings for supporting the rotor and centering it. The regulation of such a system requires an elaborate servo loop.

SUMMARY It is an object of the invention to provide a vertical axis turbomolecular pump in which the rotor supporting and centering mechanism is improved with respect to the prior art mechanism particularly in that it combines centering accuracy and. simplicity in design. It is a more particular object of the invention to provide a pump in which the rotor is centered by a part of magnetic bearings located one on each side of the rotor, while the weight of the complete rotating assembly of the pump is taken by machanical bearing means located under the rotor.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 2 is a detail view on an enlarged scale show-.

ing the upper magnetic half-bearing of the pump of FIG. 1

- FIG. 3 is a view on an enlarged scale showing the lower portion of a pump which constitutes a mbdified embodiment of the invention, this view being in crosssection along thevertical mid-plane and showing in particular the thrust-bearing.

The turbomolecular pump which is illustrated in FIGS. 1 and 2 comprises a housing lo'formed of a number of assembled parts and having an upper opening 11 which is connected to a pipe for drawing gases from the enclosure to be evacuated and a lower opening 12 which provides a connection with a pipe 14 for discharging said gases to a primary vacuum source. A vertical rotor 16 which provides a separation between the openings 11 and I2 is placed within the pump housing which has a cylindrical shape anda vertical axis. Said rotor is made up of stacked discs 18 which are mountedon a sleeve 20 and retained by a lower end-plate 22.

The pump housing 10 is also adapted to carry a stack of stationary circular discs 28 which are interposed between the rotary discs 18 and clamped between an annular shoulder 30 of the pump housing and elastic retaining members as represented diagrammatically in FIG. 1 by a simple washer 32. The longitudinal and radial clearances which are necessary for good operation of the pump are provided between the rotary discs and the stationary discs 28. These general arrangements need not be described further since reference can be made to the prior literature and in particular to US. Pat. No. I 475 765 which has already been cited.

The upper end portion of a supporting and driving shaft 34 is secured to the end-plate 22 of the rotor 16.,

Said shaft is driven in rotation by an electric motor 36 which is placed beneath the rotor 16. The motor 36 comprises a stationary assembly which is essentially constituted by a stator winding 92 carried by a casing 93 which is mounted on the base of the pump housing 10 and by a rotary assembly. Said rotary assembly is essentially made up of 'a rotor winding 94 which is carried by a sleeve 96.

In the embodiment which is illustrated in FIG. 1, provision is made for a circulation system which serves to cool the electric motor. Said system comprises a fluid supply pipe (arrow f), a jacket 98 which defines with the casing 93 an annular chamber which surrounds the stator winding 94 and a delivery pipe 5 (arrow 1' the heat which is'generated in the stator winding passes through the jacket into the coolant fluid.

The rotary assembly of the electric motor 36 is carried by a thrust-bearing which also accommodates the vertical stresses produced by the rotor 16 and transmitted by the shaft 34 (arising in particular from the weight of the rotor). Said thrust-bearing is made up of two ball-bearings 38 and 38'. The bearing 38 is interposed between the upper extremity of the sleeve 96 and a flange plate 40 which isplaced at the top portion of the casing 93. The second ball-bearing is interposed between the lower extremity of the sleeve 96 and a socket which is attached to the end-plate 42 of the casing 93. A sump 44 located beneath the end-plate 42 serves to collect any oil leakage which may develop.

' The lower portion of the shaft 34 is provided with a boss 46 and below this latter with an extension 48 of smaller diameter. Said extension'is adapted to engage within a bore having a diameter which corresponds to the sleeve and is secured in position within this latter by means of nuts 50. That portion of the shaft 34 which is located above the boss 46 passes through the sleeve within a passage having a diameter which is sufficient to permit slight bending of said shaft. Finally, the upper poition of the shaft has a cone' 52 for receiving the endplate 22 of the rotor 16.

The rotor 16 which is carried by the ball-bearings 38 and 38' is centered by means of two magnetic halfbearings 54 and 54' which are placed respectively above and below the rotor. These two bearings are of identical constructional design and are both placed between the rotor 16 and the pump housing so that only the first bearing will therefore be described.

The upper half-bearing 54 (shown in FIG. 2) comprises astationary portion which is rigidly fixed to the cover 56 of the pump housing 10 and a rotary portion which is rigidly fixed to the rotor 16. The portion which is secured to the cover 56 comprises an annular flangeplate 62 which is rigidly fixed to the cover 56 by means of threaded rings 64. The upper end portions of a number of small columns 66 which are parallel to the axis of the rotor and are three in number, for example, are fixed at uniform intervals around the periphery of the flange plate 62. The lower end of each column is attached to a casing 68 formed of a plurality of assembled parts. Said casing contains a number of magnets 72 (six magnets by way of example). The magnetic flux of said magnets is closed by means of a circuit comprising two plates 74 and 76 provided with oppositely-facing centering teeth. The rotary portion of the magnetic centering half-bearing 54 is essentially constituted by the plate 76, the teeth of which are intended to be located in alignment with the teeth of the plate 74.

The lower half-bearing 54' is similar in constructional design to the upper half-bearing and is fixed on a flange plate 78 which is secured to the pump housing by means of small columns 66.

The magnetic bearings are characterized by a relatively low degree of stiffness. It is in order to take this property into account that the stationary portions of the two half-bearings 54 and 54' are not rigidly mounted on the pump housing but are mounted by means of small columns 66 or 66 formed of material having a high degree of elasticity (non-magnetic spring steel, for example). The presence of said columns facilitates transition of the rotor to critical speeds.

The action of said small columns 66 and 66' can be completed by elastic supporting elements. Each of these elements (shown in FIG. 2) comprises a spring 80 interposed between a stud 82 which is carried by the casing of the corresponding half-bearing and an adjusting screw 84 which is attached to the flange plate 62.

It is apparent that, by means of the assembly which has just been described, two different units are intended to perform the respective functions of transmission of axial stresses and of centering. By virtue of the fact that the shaft 34 is of substantial length between the point of attachment to the sleeve 96 (centering of this latter must clearly remain within the tolerance range specified in order to ensure good operation of the electric motor) and the point of attachment to the rotor and the fact that the shaft is slightly flexible, radial displacement of the rotor is consequently permitted. Centering is thus carried out solely by the mangetic bearing which is constituted by the two half-bearings 54 and 54', each half-bearing being intended to retain one of the opposite faces of the rotor 16.

The ball-bearings illustrated in FIG. 1 are of the permanently-lubricated type and therefore do not require any additional lubrication. However, in order to prevent any oil vapour from escaping towards the rotor, provision is made for a bell-housing 86 which is fixed on the flange plate 40 and allows sufficient clearance for the radial displacements of the shaft 34.

The embodiment which is illustrated in FIG. 3 differs from the preceding embodiment primarily in the fact that it makes use of lubricated bearings. In FIG. 3 in which the components corresponding to those already illustrated in FIGS. 1 and 2 bear the same reference numerals followed by the index a, there is again shown a pump housing 10a, and end-plate 95a and a casing 93a which is carried by said end-plate and provided with a cooling system 98a. The shaft 34a which supports the rotor 16a is secured to the lower portion of a sleeve 96a. Said sleeve is centered by means of two ballbearings 42a and 38a which form a thrust-bearing. A lubrication system is associated with said bearing and has an admissionpassage 100 which opens into a space defined at the upper end of said passage by two sealing rings 102 and 104 which are adapted to cooperate so as to constitute a labyrinth seal. The inner ring 102 is attached to the sleeve 96a whilst the outer ring 104 is attached to the cover 78a which closes the casing 93a and carries the outer raceway of the ball-bearing 38a.

Oil which is admitted through the passage 100 flows downwards through the ball-bearing 38a, is projected outwards by a centrifugal deflector 106 which protects the motor, flows through a bore 108 towards the lower ball-bearing 42a and thence into the sump 44a.

Other embodiments can obviously be devised and it must be understood that any alternative forms of either all or part of the arrangements herein described which come within the definition of equivalent mechanical means are covered by this patent. In particular, the two magnetic half-bearings can be of a type other than that which has been described by way of example.

We claim l. A vertical turbomolecular pump comprising a stationary housing provided with an inlet and an outlet; a driving motor having a stationary part and a part rotatable about a vertical axis; a turbomolecular pump rotor located above said motor and coaxial therewith, separating said inlet and outlet; a transmission shaft drivably connecting said rotor and rotatable part, and means for supporting and centering said rotor, shaft and rotating part, with respect to said housing, having a pair of magnetic centering bearings located one at each axial end of said rotor, and mechanical thrust bearing means located under said rotor.

2. A pump in accordance with claim 1, wherein the thrust-bearing means comprises' two ball-bearings located one at each end of the rotating part of said motor.

3. A pump in accordance with claim 1, wherein said rotating part includes a central sleeve carried by said thrust-bearing means and wherein said shaft is secured to said rotor and to said sleeve at locations separated with a free length which imparts to said shaft a flexibility allowing radial displacement of the rotor.

4. A pump in accordance with claim 3, wherein said rotor and said sleeve are located under the rotor, the lower end of said shaft is secured to the lower portion of said sleeve, said shaft extends through said sleeve with a radial clearance and the upper end of said shaft is attached to the rotor.

5. A pump in accordance with claim 1, wherein each of said magnetic nearings comprises a rotating toothed plate securely connected to an end face of the rotor and a stationary casing provided with magnets and with a toothed plate operatively associated with said rotating plate.

6. A turbomolecular pump in accordance with claim 1, wherein said outlet is for connection with a primary vacuum source and is so located that said'motor is immersed in'the primary vacuum.

motor from the rotor. 

1. A vertical turbomolecular pump comprising : a stationary housing provided with an inlet and an outlet; a driving motor having a stationary part and a part rotatable about a vertical axis; a turbomolecular pump rotor located above said motor and coaxial therewith, separating said inlet and outlet; a transmission shaft drivably connecting said rotor and rotatable part, and means for supporting and centering said rotor, shaft and rotating part, with respect to said housing, having : a pair of magnetic centering bearings located one at each axial end of said rotor, and mechanical thrust bearing means located under said rotor.
 2. A pump in accordance with claim 1, wherein the thrust-bearing means comprises two ball-bearings located one at each end of the rotating part of said motor.
 3. A pump in accordance with claim 1, wherein said rotating part includes a central sleeve carried by said thrust-bearing means and wherein said shaft is secured to said rotor and to said sleeve at locations separated with a free length which imparts to said shaft a flexibility allowing radial displacement of the rotor.
 4. A pump in accordance with claim 3, wherein said rotor and said sleeve are located under the rotor, the lower end of said shaft is secured to the lower portion of said sleeve, said shaft extends through said sleeve with a radial clearance and the upper end of said shaft is attached to the rotor.
 5. A pump in accordance with claim 1, wherein each of said magnetic nearings comprises a rotating toothed plate securely connected to an end face of the rotor and a stationary casing provided with magnets and with a toothed plate operatively associated with said rotating plate.
 6. A turbomolecular pump in accordance with claim 1, wherein said outlet is for connection with a primary vacuum source and is so located that said motor is immersed in the primary vacuum.
 7. A pump in accordance with claim 6, having means for circulating a fluid coolant along the housing for cooling the motor.
 8. A pump in accordance with claim 7, having sealing means which separates the thrust bearing and the motor from the rotor. 